This invention relates to a millimeter wave antenna using nonradiative dielectric waveguide (NRD guide) used in particular, for example, in a collision-warning radar for an automobile.
Prior art millimeter wave NRD guide antennas are shown in FIGS. 8 and 9. FIG. 8 shows a NRD guide comprising planar conductors 51 and 52 sandwiching therebetween a dielectric strip 53. An antenna 54 is formed by connecting a transceiver circuit (not shown) at one end of the NRD guide and causing the dielectric strip 53 to protrude from between the conductors 51 and 52. The antenna 54, thus formed, is adapted to emit electromagnetic waves in the longitudinal direction of the dielectric strip 53 (in the direction of the X-axis in FIG. 8).
FIG. 9 shows a planar antenna 66 comprising a dielectric strip 61 having notches 61a formed in the direction of its width (in the direction of the X-axis in FIG. 9) for radiating electromagnetic waves and a coaxial line 62 attached to its side surface near one end for feeding in power. A NRD guide structure is formed by attaching planar conductors 63 and 64 on the upper and lower surfaces of the dielectric strip 61, and the antenna 66 is formed by providing a plurality of slots 65 to the conductor 63 on the upper surface in the longitudinal direction of the dielectric strip 61 (in the direction of the Y-axis in FIG. 9). The planar antenna 66, thus formed, is adapted to radiate electro-magnetic waves from the dielectric strip 61 in the horizontal direction (in the direction of the X-axis in FIG. 9). Electromagnetic waves are radiated through the slots 65 in the perpendicular direction to the planar antenna 66 (in the direction of the Z-axis in FIG. 9) if the slots 65 are formed in the conductor 63 with intervals therebetween equal to its wavelength.
If the prior art antenna 54 is used in a collision-warning radar and is to be attached in such a way that the direction of electromagnetic waves radiated therefrom will coincide with the direction of motion of the automobile, the antenna 54 must be disposed parallel to the direction of motion of the automobile because it can radiate electric waves only in the longitudinal direction of its dielectric strip 53 (in the direction of the X-axis in FIG. 8). Since this may cause the conductors 51 and 52 of the antenna 54 to protrude, for example, into the engine room, there is a severe limitation as to where the antenna 54 can be attached.
As for the prior art planar antenna 66, since it is used in the millimeter waveband, a high level of precision is required in the production of the notches 61a in the dielectric strip 61 and the slots 65 of the conductor 63. Since a plurality of slots 65 are required, furthermore, the surface area of the conductors 63 and 64 becomes large, and if the conductors 63 and 64 are not sufficiently strong, the conductors 63 and 64 tend to bend, causing fluctuations in antenna characteristics. Since the gain of the antenna 54 depends on the length of the dielectric strip 53, furthermore, antenna efficiency of only 20% to 50% can be attained if the loss in the dielectric strip 53 is taken into consideration. In order to increase the gain, therefore, the planar antenna 54 must be made large.